[4V50 Gary]

You talking about Part 30 (Hammer Catch Screw) or what I would call the sear screw?

I was thinking about the sear and it works just like that on the S&W, Ruger and Python. In the DA mode, the hammer tail (tall part that sticks up) catches sear, raising it up and in doing so, cause the hammer to rotate back. Then the trigger is pressed back enough, the hammer tail will disengage from the sear, allowing the hammer which is under pressure from the mainspring (via the stirrup) to rotate forward. When the trigger is released, the tail is lowered and in a Colt, the trigger tail pushes the DA sear back and out of the way so the tail can rest between the DA sear and the part of the hammer that is above its (meaning the hammer) SA sear notch.

In the SA mode, the hammer's tail holds back the SA sear notch on the hammer. The SA sear notch looks very much like the SA sear notch on a S&W or Colt revolver. Pressure on the trigger causes the two to disengage; but in the case of this problematic Webley, the hammer's fall is intercepted and preventing the haammer from rotating fully forward.